The manufacture of high molecular weight polyester polymers typically requires a two-step process. The first step, melt polymerization, reacts the monomers in the melt to achieve a relatively low molecular weight, usually amorphous polyester polymer having an intrinsic viscosity up to no more than about 0.6. This is followed by a second step where the polymer is solid-state polymerized to a high molecular weight with an intrinsic viscosity greater than 0.6. After melt polymerization, the amorphous polymer is generally formed into the shape of pellets or chips to achieve favorable solid-state reaction rates.
In order to solid state polymerize the amorphous polymer and achieve higher molecular weight, crystallinity, and a higher intrinsic viscosity, it is necessary to health the amorphous pellets or chips from room temperature to above their crystallization temperature but below their melt temperature. Once the pellets crystallize, they loose their tendency to stick together. During the course of the heat up cycle to reach the crystallization temperature of the polyester polymer, it is desirable to maintain the chips or pellets as discrete units and avoid having them stick together to both prevent plugging the reactor vessel and to manufacture a polyester polymer with a more uniform molecular weight distribution. If the pellets stick or clump together, it is not possible to proceed ahead at higher temperatures with a solid-state polymerization process and obtain a commercially acceptable product.
The problem of sticking and agglomeration of polyethylene naphthalate (PEN) pellets during the heat up cycle was described in U.S. Pat. No. 5,523,361. This patent addressed the sticking problem specifically associated with PEN polymers. Because the crystallization temperature of the PEN polymer is so high (180-200.degree. C.), the patent teaches that the PEN polymer passes through its glass transition sticking temperature (about 140.degree. C.) prior to crystallization, resulting in sticking and clumping. The patent teaches that the solution to this problem was to coat the PEN polymer with an alkylene carbonate such as ethylene carbonate or propylene carbonate prior to crystallization. The carbonate-coated PEN pellets crystallize more rapidly and at a lower temperature than uncoated PEN, reducing or eliminating the tendency of the pellets to stick together during the crystallization process.
PEN polymers are not the only polymers which stick and clump together during the crystallization heat up cycle of the solid-state polymerization process. Like PEN polymers, polyethylene isophthalate (PEI) polymers also stick and clump during the crystallization heat up cycle because the glass transition temperature of PEI is quite low, about 50.degree. C. However, unlike PEN polymers, we have found that coating alkylene carbonates, such as ethylene carbonate, onto PEI would not prevent PEI from sticking during its heat up cycle. Sufficient surface crystallinity on PEI could not be induced to prevent sticking above 50.degree. C. Thus, unlike PEN polymers, coating an alkylene carbonate on PEI was not a solution to avoid sticking. The problem of sticking during the heat up cycle in solid-state polymerization could not be solved by looking to the state of the art associated with polyethylene terephthalate (PET) because this polymer typically does not exhibit a sticking problem during the crystallization heat up cycle in the solid-state process.
It would be desirable to utilize the solid-state process with polyalkylene isophthalate polymers (PAI) for numerous applications because their barrier resistance to atmospheric moisture and oxygen are 2 to 8 times better than PET polymers. However, to commercially manufacture a polyester polymer containing PAI, and specifically PEI, the sticking problem must first be overcome.